Thermoplastic foams made from alkenyl aromatic polymers (e.g. polystyrene) or polyolefin polymers (e.g. polyethylene and polypropylene) have found extensive use, particularly as packaging and insulating materials. Such foams are commonly manufactured as expanded beads, extruded sheets, or extruded boards. The difference between the expanded and extruded foams is that the extruded foams in the form of continuous sheets or boards, are made in a single-step process; whereas, expanded foams, in the form of discrete, small-size pieces, are made in a multi-step process. Thus, the dimensions of expanded foam are much smaller than those of extruded foam. Furthermore, the expanded foams do not necessarily have to be in the form of beads or peanuts, but can also be made from pellets, rods, platelets, thin sheet or film. For the sake of convenience, the term “bead” or “pellets” will be used throughout this application to imply other shapes in which small, discrete particles of the polymer resin can be used to make expanded foams.
Generally, alkenyl aromatic polymer foams in the form of beads or sheets having a thickness of less than about one-half inch are used to make packaging materials such as containers (e.g. cups, bowls, clamshells, picnic chests) for hot or cold beverages or food, and for protection during transportation of delicate or shock sensitive articles whereby the beads are fused or the sheet is thermoformed in a mold to give the packaging material of a desired shape. The foam beads are also used as loose fill dunnage material. Generally, insulating foams are produced in thickness greater than about one-half inch. The insulating value of such foams is measured in terms of heat conduction resistance or R-value, per one inch of foam thickness. Adequate insulating foams typically have R-values of about 4.0 or greater.
Packaging and insulation foam products with thickness greater than about 0.5 inch are called planks or boards. Such foam boards are produced in the desired shape and size by direct extrusion and cutting if needed, or by fusing the expanded foam beads. The foam boards can be used for protective packaging by die-cutting the boards to various shapes, for insulation, for dissipating mechanical energy as in automotive parts, or for cushioning floats. It is desirable that foams are dimensionally stable; this characteristic is even more desirable for planks or boards.
Polymer foams are commonly made using a continuous process where a blowing agent laden molten resin is extruded under pressure through an appropriate die into a lower pressure atmosphere. Alternatively, a batch or staged process can be used, where small polymer beads (also called particles or pellets) are impregnated with blowing agent and then heated rapidly to a temperature near or above the glass transition temperature of the polymer-blowing agent system, or subjected to an external compressive stress at a temperature up to the glass transition temperature of the polymer-blowing agent system.
In the past, physical blowing agents widely used for making foams were chlorofluorocarbons (CFCs) which, because of their high ozone depletion potential (ODP), were subsequently replaced with hydrochlorofluorocarbons (HCFCs) with ODP values much smaller than those of CFCs. Notwithstanding their reduced ODPs, the HCFCs are scheduled to be phased out by the year 2010 in the United States, and are increasingly being replaced with hydrofluorocarbons (HFCs), the latter having a) zero ODP, thereby minimizing damage to the ozone layer; and b) a thermal conductivity lower than most polymers or blowing agents, thereby lowering the foam's thermal conductivity.
Presently, physical blowing agents more commonly used for making thermoplastic polymer foams such as alkenyl aromatic polymer (e.g. polystyrene) or polyolefin polymer (e.g. polyethylene or polypropylene) foams are hydrocarbons, chlorinated hydrocarbons, HCFCs, HFCs, or combinations thereof. Hydrocarbons with three or more carbon atoms are considered volatile organic compounds (VOCs) that can lead to formation of smog. Furthermore, some halogenated hydrocarbons are either VOCs or have high ODP or are hazardous air pollutants (HAPs) and, at times, may fall into more than one of these categories. Therefore, the use of hydrocarbon and halogenated hydrocarbon blowing agents for preparing polymeric foams is not preferred environmentally and imposes many limitations on the manufacturing process, thus complicating and significantly increasing the cost of manufacturing. For example, alkenyl aromatic polymer (e.g. polystyrene) packaging foams (beads or sheets) are generally made using VOCs such as butanes or pentanes, and insulating foams are currently made using VOCs such as hydrocarbons and halogenated hydrocarbons or non-VOCs such as 1-chloro-1,1-difluoroethane (HCFC-142b), alone or in combination with ethyl chloride, which is classified both as a VOC and a HAP. It is therefore desirable to minimize or eliminate altogether the use of VOC and/or HAP compounds as blowing agents for preparing polymeric foams.
Methyl formate is classified as a non-VOC (Federal Register, Volume 69, Number 228, Nov. 29, 2004), is non-HAP, has zero ODP, and negligible global warming potential (GWP). U.S. Pat. No. 6,753,357 to Kalinowski, which is incorporated in its entirety herein by reference thereto, describes the use of methyl formate to produce stable, rigid isocyanate/polyol based polyurethane foams. It is noted, however, that such polyurethane foams are thermoset, so as to be made via a cross-linking and curing process. The dimensional stability or instability imparted to the final polyurethane foam product by the nature of the blowing agent therefore is quite different than in the case of thermoplastic polymer foams.
U.S. Pat. No. 3,914,191 to Scott, which is incorporated in its entirety herein by reference thereto, describes the use of a minimum boiling azeotropic mixture of 18 percent by weight (wt %) methyl formate and 82 wt % trichloromonofluoromethane (CFC-11). The stability of the foam is attributed to the minimum boiling characteristics of the azeotropic mixture, which generates a higher cell pressure—as opposed to a maximum boiling azeotropic or a non-azeotropic mixture—and, thereby, prevents cell collapse. Thus, Scott does not teach the use of compositions other than minimum boiling azeotropic compositions to make stable foams.
U.S. Pat. No. 3,900,433 to Taub, which is incorporated in its entirety herein by reference thereto, describes a styrene polymer bead impregnated with a blowing agent blend containing from about 0.5 to about 20 wt % of an impregnation aid. Particularly, Taub discloses a blowing agent blend including dichlorofluoromethane and methyl formate as the impregnation aid. The methyl formate when present in an amount of 3 wt % of the blowing agent blend (corresponding to 0.084 wt % of the overall composition (polymer and blowing agent blend and additives)) was found to give free flowing beads. However, when the methyl formate is present in an amount of 6 wt % of the blowing agent blend (corresponding to 0.44 wt % methyl formate of the overall composition (polymer and blowing agent blend and additives), it resulted in beads that tended to fuse together.
U.S. Pat. No. 4,098,941 to Johnson, which is incorporated in its entirety herein by reference thereto, describes a process for quenching extrudate in boiling water to control expansion and form a density gradient from the core to the surface of the extrudate. Methyl formate is included in a list of possible volatile liquids that can be used in this process.
U.S. Pat. No. 4,104,440 to Collins, which is incorporated in its entirety herein by reference thereto, is directed to a melt extrusion process where the surface of the expanding foam is quenched to produce a substantially unfoamed “skin.” Methyl formate is included in a list of possible volatile liquids that can be used in this process.
U.S. Pat. No. 5,532,284 to Bartlett, which is incorporated in its entirety herein by reference thereto, is directed to closed cell thermoplastic or thermoset polymer foam and process for manufacturing thereof, with a gas barrier resin substantially uniformly dispersed in the continuous polymeric phase which reduces the permeability of the foam, thereby maintaining a blowing agent in the cells of the foam. Methyl formate is included in a list of possible blowing agents.
U.S. Pat. No. 6,315,932 to Fujiwara, which is incorporated in its entirety herein by reference thereto, is directed to a melt extrusion process for producing polystyrene insulation boards, where the blowing agent mixture includes 5 to 40 wt % of dimethyl ether, diethyl ether and/or methyl ethyl ether, and 60 to 95 wt % of saturated C3 to C5 hydrocarbon. Optionally, additional blowing agents can be added to the blowing agent mixture, such as, fluorinated hydrocarbons, organic gases and carboxylic acid esters, such as methyl formate.
U.S. Pat. Nos. 6,569,912 and 6,762,212 to Oohara, which are incorporated in their entirety herein by reference thereto, are directed to an extruded polystyrene board obtained by a melt extrusion process, where the blowing agent mixture includes 10 to 100 wt % of a saturated C3 to C5 hydrocarbon and 0 to 90 wt % of a co-blowing agent. Methyl formate is included in a list of possible volatile liquids that can be used in this process. However, Oohara's patents further require specific combinations of flame retardants to be used in the process. The flame retardants, include, for example, a halogenated flame retardant and at least one compound including, phosphorus type flame retardants, tetrazole compounds, cyanuric acid, isocyanuric acid, metal borates and boron oxides.
Therefore, a need exists for blowing agent blends employing methyl formate and environmentally friendly co-blowing agents, preferably non-VOC and/or non-HAP co-blowing agents. A further need exists for efficient, cost-effective processes for producing dimensionally stable thermoplastic foams, without compromising the product quality or performance characteristics in terms of appearance, thermoformability, mechanical or compressive strength, resistance to flame-spread, and insulation value, utilizing blowing agent blends employing methyl formate in particular combinations and/or amounts that do not pose constraints on the thermodynamic state (i.e. azeotropic or not) of the blowing agent blend.